Method of forming a skinned polyurethane foam by overfilling a closed preheated mold

ABSTRACT

A process for the production of rigid polyurethane articles with a hard dense skin which comprises charging a mold with liquid polyurethane foam components in a amount equal to twice but not exceeding ten times the quantity which would be required to give the same volume as the mold cavity were the foam allowed to free blow, said mold having been preheated to a temperature of from 110* to 175* F. before charging; closing the mold; curing said foam while externally heating to at least maintain the preheat of the mold; and removing the article from the mold.

United States Patent [72] Inventors Victor G. Soukup;

Donald Dunn, both of Cincinnati, Ohio 2| 1 Appl. No. 759,765

[54] METHOD OF FORMING A SKINNED POLYURETHANE FOAM BY OVERFILLING ACLOSED PREHEATED MOLD 6 Claims, No Drawings [52] US. Cl 264/45, 260/25AZ, 264/48, 264/DlG. 14 [51] Int. Cl B29d 27/04 [50] Field of Search264/54, 48, 45; 260/25 AZ [56] References Cited UNITED STATES PATENTS 3125,617 s 19m H oppe -264/54 3,178,490 4/1965 Petrino. 264/41 3,l82,l045/1965 Cwik 264/45 OTHER REFERENCES Primary Examiner'Donald J. ArnoldAssistant Examiner- Paul Leipold Attorney- Plumley, Tyner & SandtABSTRACT: A process for the production of rigid polyurethane articleswith a hard dense skin which comprises charging a mold with liquidpolyurethane foam components in a amount equal to twice but notexceeding ten times the quantity which would be required to give thesame volume as the mold cavity were the foam allowed to free blow, saidmold having been preheated to a temperature of from 1 10 to 175 F.before charging; closing the mold; curing said foam while externallyheating to at least maintain the preheat ofthe mold; and removing thearticle from the mold.

FOAM av OVERFILLINGA caosaornananmn} e MOLD I This. application-is acontinuation-impart of Ser. No.

489,037flledSept.2l,1965. a

This invention relates to methods of preparing foamed plastic materialsand more'particularly to methods of producing improved molded articlesof polyurethane foams.

'Foamed plastic materials have foundwide and increasing applicability infields wherein their properties, such as light weight, relatively highstrength to weight ratio, buoyancy, and low thermal conductivity providedistinct advantages over other materials. Foamed polyurethanesbecause oftheir low cost and. goodphysical properties have been proposed for amultitude of end uses.

- Polyurethane foams, in common with other foamed plastics, possess acellular structure, the exterior surfaces of which tend to have a rough,irregular character which is undesirable and, more importantly, becauseof the relative sofiness of this surfaces of this cellularstructure,.molded foamedarticles have often times been found so lackingin physical strength and resistance to abrasion as to limit theirusefulness.

in efforts to overcome these disadvantages, many and various moldingtechniques and manipulations of the foamable plastic'com ponents havebeen proposed in the past, the main objective being toobtain a smoothskin or surface on the exterior of the molded articles so as to improvetheir abrasion resistance and to' add to their structural strength.Among these methods are included techniques for adhering metallic andnonmetallic skins to the foams. ln' addition, it has been proposed tosubject the molded article to heat and compacting pressureto effect amelting and hence smoothing of the surface. Such a procedure has amongits disadvantages the fact that it presents an added step whichincreases the cost of preparing the final article.

Again, proposals have been made to insulate the mold walls topreventradiation of heat during the molding step thus allegedly allowingentrapment of the gas evolved from the foaming material in theperipheral cells andsealin'g of each cell from the other, thus forming asmooth skin over the exposed surfaces. It has alsov been proposed tosubject the foaming material in selected surface areas to moldtemperatures below the effective vaporization temperature of the blowingagent so as to avoid foaming and to effect a polymerization of the foamreactants in those selected'places and thus obtain a smooth skin orsurface in such areas.

These and many other similar proposals of the prior art have the commondisadvantage, among others, of adding procedu-' ralsteps and complexequipment which, in turn. add to the time, effort, and cost of theoverall molding operation and resultant product.

It is an object of the present invention to overcome the disadvantage ofthe prior art and to provide a method for the production of new andimproved molded articles from polyurethane foam-forming materials.

Other objects and advantages will be apparent from the followingdescription.

It has been found, according to this invention, that if mixed liquidpolyurethane foam components are charged into a previously heated, fixedvolume, closable mold in an amount at least twice that which would berequired to give the same volume as that of the mold cavity were thefoam components allowed to free blow, and if, thereafter, the chargedcomponents are allowed to foam and are thereafter cured, a moldedpolyurethane foam article is obtained which completely fills the moldand has an overall density at least l percent greater than the free blowdensity of the same foam formulation and has greatly improvedcharacteristics such as high structural strength and a dense, hard,smooth skin or layer on the exterior surfaces of the molded article.

The process of this invention yields a polyurethane foam moldingcomprising a cellular core totally enclosed by a self generated, dense,relatively hard, durable skin layer of essen- 75 rethane foam system,and which can thermally decompose to I tially the same chemicalcompositionas the core and having a D ID, ratio of from 0.30m 0.90 and aDJD, ratio of from 1.2 to 3.0 whereD, is the density of thecore, D, isthe density of the skin, and D, is the overall density of the foammolding; (In

.nonmineral tilled" polyurethanes, D, does not exceed about 70 lbs. percubic footand the ratios D,./D, and DJD, must not be used in such manneras to obtain values of D, greater than 70 lbs. per'cubic foot.)Typically polyurethane foam moldings obtained bythe processof thisinvention have been found to have structural utility from the standpointof factors such as high strength to weight ratios, high stiffness, highimpact strength, high surface hardness, and durable outersurfaces and toexhibit overall densities in the range of 15.0 to 40,0 lbs. per cubicfoot and preferably in the range of 18.0 to 30.0 lbs. percubic foot. I

It has been found that the ratio between the amount of mixed foamcomponents added to the mold and the amount of mixed foam componentswhich would free blow to the same volume as that of the mold cavity is acritical and controlling factor and must be carefully observed in orderto fill the mold completely and obtain the improved products of thisinvention.

From a practical standpoint, this invention makes it possible for thefirst time to determine in advance the amount of charge of polyurethanefoam component'of known free-blow density .which need be placed in amoldof knownvolume to give an article of any desired overall density frompercent of the ultimate'free blow density upwards,'and which articlehasa hard, dense, smooth skin on its exterior surfaces. Thus; forexample, in case of a mold having a volume of 1 cubic foot, if oneemploys a polyurethane foam having a'free-blow density of 5 pounds percubic foot and desires a molding of 20 pounds per cubic foot overalldensity, one would place 20 pounds of foam components in the mold toachieve a packing-factor of 4, said packing factor beingthe controllingfeature in obtaining the resultant improved products of this invention.7

Shelves, drawers, and similar articles having commercial value andutility have been made by the process of this invention and the hard,dense skin and relatively high overall density of such products fit themfor a multitude of similar end? uses. 1

It has also been found that highly uniform and-desirable results areobtained, in the practiceof this invention, if ,one end of thelongestdimension of the mold is elevated at least slightly from thehorizontaL'lt is believed that this permits more rapid and completeexpansion "of the foam to all areas within the mold cavity.

The practice of this invention is adaptable to the use of polyurethanefoam systemsgenerally. Thus, there may be em ployed in the practice ofthe invention polyurethane foams which may be prepared by numerousmethods and materials. Such methods and materials include: those whereinall of the components such as the polyisocyanate, polyol, catalyst,blowing agent, and surfactant are mixed together immediately prior tobeing added to a mold; those wherein the polyisoa cyanate component isprereacted with a portion of the polyol component to form an isocyanateterminated prepolymerof either relatively low or relatively highmolecular weight and said isocyanate terminated prepolymer mixed withthe remainder of the polyol component in the presenceof catalyst,blowing agent, and surfactant immediately prior tobeing added to a mold.The formation of polyurethane foam may be accomplished by blowingmechanisms involving: (a)

the thermal decomposition of one or more components to.

a of said polyurethane foam system. Numerous compounds,

which can be present as oneor more components of a polyuyield a gaseousproduct or products, thereby causing expansion of the foam, are cited inthe polyurethane foam literature and patents. Several compounds of thistype which will decompose with liberation of a gas and which have beenshown to be useful for foam formation are N,N-dinitrosopentamethylenetetramine, azobisisobutyronitrile, N,Ndimethyl-N,N-dinitrosotetephthalamide, diazodiaminobenzene, oxybisbenzenesulfonyl hydrazide, and the oxalate of dicyandiamide. Volatilization ofan inert low boiling component of the polyurethane foam formulationconstitutes another method for achieving foam formation. Suitablecompounds capable of volatilization to produce a foam structure whichhave been previously used include trichlorofluoromethane,dichlorodifluoromethane, pentane, isopentane, chloroform, and methylenechloride. The choice of the low boiling inert 1 liquid for foaming isoften dictated by factors such as flammability and toxicity. As a resultof these considerations, low boiling halogenated hydrocarbons aregenerally preferred for producing polyurethane foam via this method,viz, the volatilization of an inert, low boiling component. Reactionsbetween isocyanates and water, isocyanates and carboxylic acids,isocyanates and inorganic acids, such as hydrochloric acid, andisocyanates and alkali metal carbonates, such as sodium carbonate, canbe used to generate carbon dioxide which in turn will cause expansion ofthe polyurethane system to produce a foam structure.

While polyols (organic polyhydroxyl compounds) are generally used in thepreparation of polyurethane foam, other organic molecules containing twoor more Zerewitinoff-active hydrogen atoms, may be employed.Polyisocyante compounds (organic molecules having two or more isocyanategroups) employed in the preparation of polyurethane foam may bealiphatic, aromatic, heterocyclic, or aryl-alkyl in nature. A multitudeof such known components and methods for the production of polyurethanefoam components and the mixing thereof are given in the literature suchas in U.S. Pat. No. 3,127,457, 3,154,606, and Reissue 24,514.

The fixed volume, closable mold into which the charge of foam componentsis placed is desirably heated, prior to the charge, to a temperature (ofat least about in the range of 1 to 175 F.), preferably about 120 F.After closing the mold, foaming is allowed to take place for an intervalof l to 4 minutes, during which the mold is at a temperature about orslightly greater than that of the foam components added to the mold.Following completion of this foaming reaction, i.e., expansion to form acellular product in the closed mold, which normally takes place in 1 to4 minutes, the foamed material in the closed mold is cured, e.g., bymaintenance of the heat ofor addition of heat to the mold, this heatinginterval being dependent upon the type of heat applied. Thus, inductionor high frequency heating may be accomplished in seconds while moreconventional infrared, hot air, or like heating may require from 10-30minutes. The resulting product is found to have a smooth, hard,abrasion-resistant skin of relatively high density on its outersurfaces, totally enclosing and integral with an inner, more cellularstructure, the overall density of the product being at least 100 percentgreater than the free-blow density. It is thus possible, according tothis invention, to prepare articles of such strength, uniformity, andexteriorly smooth surfaces that they are adapted for a multitude ofend-uses.

The following examples will serve to illustrate in greater detail howthis invention may be practiced.

EXAMPLEI 100 grams of a polyol mixture at about 106 F. comprising 97.0grams of a phosphoric/phosphorous acid ester polyhydroxyl compound ofhydroxyl number of about 460 Pelron" 9744 made by Pelron Corp.), 1.0grams of a silicone surfactant, and 2.0 grams of dimethylethanol amineare mixed thoroughly with 138 grams of an isocyanate prepolymer mixtureat about 70 F. comprising 131.5 grams of an isocyanate terminatedprepolymer having a free isocyanate content of about 25 percent and 6.5grams of monofluorotrichloromethane.

45.8 grams of the warm resulting liquid mixture while still liquid arecharged into a 12 cubic inch rigid aluminum mold (2 X1 X6 inches)previously heated to about 120 F. The mold is closed and clamped shut.About 2 minutes after closing and clamping the mold is placed in acirculating air oven at 120 F. for 20 minutes. Upon removing the moldfrom the oven the mold is unclamped and the molded part is removed. Thecompletely filled out molding obtained has an overall density of 14.6lbs./ft. 45.8 grams of the above resulting liquid mixture when allowedto free blow in an open paper cup forms a foam volume of 28.7 cubicinches and the resulting foam exhibits a density of 6.1 lbs./ft. 19.9grams of the above resulting warm liquid mixture under the same moldingconditions when charged into the 12 cubic inch mold failed to yield acompletely filled out molding. Had the molding been completely filledout, it would have had an overall density of 6.3 lbs./ft. 29.0 grams ofthe above resulting warm liquid mixture under the same moldingconditions when charged into the 12 cubic inch mold failed to yield acompletely filled out molding. Had complete fill out occurred, theresulting molding would have had an overall density of 9.2 lbs/ft. 35.4grams of the above resulting warm liquid mixture under the same moldingconditions when charged into the 12 cubic inch mold failed to yield acompletely filled out molding. Had complete fill out occurred, themolding would have had an overall density of 1 1.2 lbs/fts".

Increasing the charge of the above resulting warm liquid mixture to the12 cubic inch mold to 63.9 grams and 98.1 grams produced completelyfilled out moldings, under the same molding conditions, whose surfaceproperties (surface hardness, mar resistance, etc.) increased withincreasing charge of the liquid mixture. The variation of the charge ofthe above resulting warm liquid mixture to the 12 cubic inch mold andeffect of this variation on molding results and quality of the moldingmay be seen in the following chart.

Overall density for c0mplete fill Charge to the mold Percent out (lbs./(gms fill out ftfi) Molding quality 50 6. 3 Soft; surface. ll. T? DO. 9811.2 Do. 100 14.6 Moderately hard surface. 100 20. 3 Hard surface. 10031. 3 Very hard surface.

EXAMPLE 11 Following the molding procedure and molding conditions inExample I, 100 grams of a polyol mixture, Selectro foam 6500A42-15l,made by the Pittsburgh Plate Glass Co.). at about 70 F. comprising 91.39grams of an organic polyhydroxyl compound hydroxyl number of about 456,0.53 grams of water, 1.13 grams ofa silicone surfactant, 0.39 grams ofdimethyl ethanol amine, 0.15 grams of DABCO, and 6.41 grams ofmonofluorotrichloromethane are mixed thoroughly with 1 18.3 grams ofMondur MR (crude diphenyl methane 4,4'-diisocyanate) at about 121 F.

18.0 grams of the above resulting warm liquid mixture when place in the12 cubic inch mold under the same molding conditions failed to yield acompletely filled out molding. Had the molding been completely filledout it would have had an overall density of 5.7 lbs./ft. 22.3 grams ofthe above resulting warm liquid mixture when charged to the 12 cubicinch mold under the same molding conditions failed to yield a completelyfilled out molding. l-lad complete fill out occurred, the molding wouldhave had an overall density of 7.1 lbs/ft. 46.7 grams, 60.7 grams, and89.5 grams of the above resulting warm liquid mixture when placed in the12 cubic inch mold under the same molding conditions as in thepreviously stated moldings yielded completely filled out molded partswhose surface characteristics (hardness, mar resistance, etc.) increasedwith the increasing amount of the liquid mixture placed in the mold.

EXAMPLE lll Following the molding procedure and molding conditions inExample I, 100 grams of a polyol mixture at about 106 F. comprising 97.0grams of a phosphoric/phosphorous acid ester polyhydroxyl compound ofhydroxyl number of about 460, as described in Example 1, 1.0 grams of asilicone surfactant, and 2.0 grams of dimethyl ethanol amine are mixedthoroughly with 163.1 grams of an isocyanate prepolymer mixture at about70 F. comprising 145.6 grams of an isocyanate terminated prepolymerhaving a free isocyanate content of about 25 percent and 17.5 grams ofmonofluorotrichloromethane.

25.4 grams of the above resulting warm liquid mixture when charged tothe 12 cubic inch aluminum mold previously heated to about 120 F.yielded a completely filled out molding having an overall density of 8.11bs./ft. 25.4 grams of the above resulting warm liquid mixture whenallowed to free blow in an open paper cup yields a foam volume of 24.4cubic inches and the resulting foam exhibits a density of 4.0 lbs/ft.19.8 grams of the above resulting warm liquid mixture when placed in the12 cubic inch mold under the same molding conditions failed to yield acompletely filled out molding. Had the molding been completely filledout, it would have had a density of 6.3 lbs./ft. 14.2 grams of the aboveresulting warm liquid mixture, when charged to the 12 cubic inch moldunder the same molding conditions, failed to yield a completely filledout molding. l-lad complete fill out occurred, the molding would havehad an overall density of 4.5 lbs./ft. 37.8 grams, 51.0 grams, 63.3grams, and 94.2 grams of the above resulting warm liquid mixture whencharged to the 12 cubic inch mold yielded completely filled out moldingwhose surface properties (hardness, mar resistance, etc. increased withincreasing charge to the mold.

EXAMPLE lV Following the molding procedure and molding conditions inExample I, 100 grams of a polyol mixture of about 160 F. comprising 98.0grams of a phosphoric/phosphorus acid ester polyhydroxyl compound ofhydroxyl number of about 460, as described in Example 1, 1.0 grams of asilicone surfactant and 1.0 grams of dimethyl ethanolamine arethoroughly mixed with 182.5 grams of an isocyanate prepolymer mixture atabout 70 F., comprising 146.0 grams of an isocyanate terminatedprepolymer having a free isocyanate prepolymer mixture at about b 70 F.,comprising 146.0 grams of an isocyanate terminated prepolymer having afree isocyanate content of about 25 percent and 36.5 grams ofmonofluorotrichloromethane. 15.1 grams of the resulting warm liquidmixture while still liquid when charged into the 12 cubic inch aluminummold previously heated by about 120 F. yielded a completely filled outmolding having an overall density of 4.8 lbs./ft. 15.1 grams of theresulting warm liquid mixture when allowed to free blow in an open papercup yielded a foam volume of 25 cubic inches and the resulting foamexhibits a density of 2.3 lbs./ft. 10.9 grams of the resulting warmliquid mixture when placed in the 12 cubic inch mold under the samemolding conditions failed to yield a completely filled out molding. Hadthe molding been completely filled out it would have had an overalldensity of 3.5 lbs./ft. 8.9 grams of the resulting warm liquid mixturewhen charged into the 12 cubic inch aluminum mold under the same moldingconditions failed to yield a completely filled out molding. Had completefill out occurred, the molding would have had an overall density of 2.8lbs./ft.". 30.4 grams and 62.1 grams of the resulting warm liquidmixture when charged to the 12 cubic inch aluminum mold under the samemolding conditions yielded completely filled out moldings whose surfacecharacteristics (hardness, mar resistance, etc.) increased with theincreasing amount of the liquid mixture placed in the mold.

What is claimed is:

l. The process for the preparation of rigid molded polyurethane foamarticles having a hard, dense, self-generated skin totally enclosing acellular core of lower density composed of essentially the same chemicalcomposition which comprises 1. charging a fixed volume, elosable moldwith mixed liquid polyurethane foam components in an amount equal totwice but not exceeding ten times the quantity which would be requiredto give the same volume as the mold cavity were said foam componentsallowed to free blow, said mold having been preheated to a temperatureof from 1 10 to F before charging; completely closing said mold;

allowing said foam component to foam to the complete volume of the moldcavity while retaining said foam within said cavity;

4. curing said foam within said cavity by externally adding heat to theextent to at least maintain the preheat of the mold; and

5. removing the resultant cured article from the mold.

2. The process of claim 1 in which the polyurethane foam is obtained bythe reaction of an organic polyisocyanate with an organic compoundcontaining at least two Zerewitinoff-active hydrogen atoms.

3. The process of claim 1 in which the polyurethane foam is formed bythe gaseous product resulting from the reaction of two components of theliquid polyurethane foam component mixture.

4. The process of claim 1 in which the resultant polyurethane foamarticle is formed by the volatilization of an inert liquid blowing agentsaid inert liquid blowing agent being nonreactive with isocyanate.

5. The process of claim 1 wherein the resultant rigid moldedpolyurethane foam article has a D lD ratio of from 0.300/l.00 to0.90/l.00 and a D,/D,, ratio of from 1.2/1.0 to 3.0/1.0 where D is thedensity of the core expressed in pounds per cubic foot or equivalentunits, D, is.the density of the skin expressed in pounds per cubic footor equivalent units and D is the overall density of the foam moldingexpressed in pounds per cubic foot or equivalent units and where D shallnot exceed 60 pounds per cubic foot for a nonmineral filled polyurethaneand D shall be from 15 pounds per cubic foot to 40 pounds per cubic footprovided the ratio of D to D in the defined range shall not be taken ina manner such as to obtain a calculated value of D, greater than theprescribed limit of 60 pounds per cubic foot for values of D, within thegiven range of 15 to 40 pounds per cubic foot.

6. The process of claim 5 wherein D is from 18 to 30 pounds per cubicfoot.

2. completely closing said mold;
 2. The process of claim 1 in which thepolyurethane foam is obtained by the reaction of an organicpolyisocyanate with an organic compound containing at least twoZerewitinoff-active hydrogen atoms.
 3. The process of claim 1 in whichthe polyurethane foam is formed by the gaseous product resulting fromthe reaction of two components of the liquid polyurethane foam componentmixture.
 3. allowing said foam component to foam to the complete volumeof the mold cavity while retaining said foam within said cavity; 4.curing said foam within said cavity by externally adding heat to theextent to at least maintain the preheat of the mold; and
 4. The processof claim 1 in which the resultant polyurethane foam article is formed bythe volatilization of an inert liquid blowing agent said inert liquidblowing agent being nonreactive with isocyanate.
 5. The procesS of claim1 wherein the resultant rigid molded polyurethane foam article has aDc/Do ratio of from 0.300/1.00 to 0.90/1.00 and a Ds/Do ratio of from1.2/1.0 to 3.0/1.0 where Dc is the density of the core expressed inpounds per cubic foot or equivalent units, Ds is the density of the skinexpressed in pounds per cubic foot or equivalent units and Do is theoverall density of the foam molding expressed in pounds per cubic footor equivalent units and where Ds shall not exceed 60 pounds per cubicfoot for a nonmineral filled polyurethane and Do shall be from 15 poundsper cubic foot to 40 pounds per cubic foot provided the ratio of Ds toDo in the defined range shall not be taken in a manner such as to obtaina calculated value of Ds greater than the prescribed limit of 60 poundsper cubic foot for values of Do within the given range of 15 to 40pounds per cubic foot.
 5. removing the resultant cured article from themold.
 6. The process of claim 5 wherein Do is from 18 to 30 pounds percubic foot.